Water-based treatment fluids can damage some formations for several reasons including clay swelling, emulsion/water block, wettability alteration and relative permeability effects. Hydrocarbon-based fluids are perceived as less damaging to hydrocarbon-bearing formations than water-based fluids. Because of several handling limitations, lack of temperature stability, cost, dependence of base hydrocarbon source and nature on gellation properties, they are not often the primary choice. However, they are used in formations that are known to be extremely water sensitive.
In the 1960s, aluminum salts of carboxylic acids (for example, aluminum octoate) were used to raise the viscosity of hydrocarbon-based fluids (Burnham, J. W., Harris, L. E. and McDaniel, B. W.: “Developments in Hydrocarbon Fluids for High-Temperature Fracturing,” paper SPE 7564, presented at the SPE Annual Technical Conference and Exhibition, Houston, Tex., USA (Oct. 1-3, 1978); also in Journal of Petroleum Technology (February, 1980) 32, No. Z 217-220). This improved the temperature stability and solids-carrying capability of the fluids and the technology was recommended for hydraulic fracturing applications. In the 1970s, aluminum carboxylate salts were replaced by aluminum phosphate ester salts. This helped in increasing the temperature range in which this oil-based fluid could be used and also enhanced the proppant transport ability of the system. Today, aluminum phosphate ester chemistry remains the preferred method of gelling hydrocarbons for fracturing purposes. Both methods of thickening oil rely on an “associative” mechanism (Baker, H. R., Bolster, R. N., Leach, P. B. and Little, R. C.: “Association Colloids in Nonaqueous Fluids,” Ind. Eng. Chem., Prod. Res. Develop. (1970) 9, No. 4, 541-54.). It is believed that the interactions between the aluminum complexes and phosphate ester molecules in these prior art fluids produce a long polymer chain as shown in FIG. 1A (Burnham et al., 1980).
The R groups shown in FIG. 1A are hydrocarbon chains and are soluble in the oil to be gelled. These soluble R groups keep the aluminum phosphate ester polymer in solution. Generally, the R groups are hydrocarbon chains containing up to 18 carbon atoms (Crawford, D. L., Earl, R. B. and Monroe, R. F.: “Friction Reducing and Gelling Agent for Organic Liquids,” U.S. Pat. No. 3,757,864 (Sep. 11, 1973). The R groups have a high affinity for oils such as kerosene and diesel that comprise 12- to 18-carbon (and somewhat higher) chains. Crude oils are composed of a larger number of different organic compounds and may contain paraffins and asphaltenes. Some high-molecular weight compounds, especially paraffins and asphaltenes, are not compatible with the aluminum phosphate ester gelling system. Many crude oils may be gelled, but testing them prior to use in the field is highly recommended.
The R groups can be pictured as forming an oil compatible shield around the polar core of aluminum ions (McKenzie, L. F. and Hughes, B. J: “Hydrocarbon Gels of Alumino Alkyl Acid Orthophosphates,” paper SPE 9007, presented at the 5th International Symposium on Oilfield and Geothermal Chemistry, Stanford, California, USA (May 28-30, 1980). Polar species (such as water, acids, bases or salts) are incorporated into the polar core and affect the association of the aluminum ions and phosphate ester groups. These materials can make the gel structure more rigid, or they can destroy the gel structure.
The viscosity of the standard aluminum phosphate ester gel is controlled by varying the quantities of aluminum compound and phosphate ester. To improve high-temperature performance, the viscosity of the gel can be increased by increasing the amount of polymer; however, this results in very high viscosities on the surface, which make it difficult to draw the fluid out of the tanks to the pumps. One approach used is to add part of the gelling materials “on the fly” so that high viscosity is not achieved until the fluid reaches the fracture (Harris, L. E., Holtmyer, M. D. and Pauls, R. W.: “Method for Fracturing Subterranean Formations,” U.S. Pat. No. 4,622,155 (Nov. 11, 1986); Cramer, D. D., Dawson, J and Ouabdesselam, M.: “An Improved Gelled Oil System for High-Temperature Fracturing Applications,” paper SPE 21859, presented at the Rocky Mountain Regional Meeting and Low Permeability Reservoirs Symposium, Denver, Colo., USA.). On-the-fly addition means that the materials are added to the fluid as the fluid is pumped downhole. Another approach is to maximize thermal stability by carefully controlling the composition of the solution to provide optimum conditions for association of the aluminum and phosphate ester species (Gross, J M: “Gelling Organic Liquids,” U.S. Pat. No. 5,190,675 (Mar. 2, 1993).
Typically, these systems take a long time to gel once the chemicals are mixed together. Recent developments in gelled oil chemistry make a true continuous-mix (all materials added on the fly) gelled oil possible. By changing the aluminum source, the aluminum/phosphate ester ratio in the gel and/or the phosphate ester mix (Daccord, G., Lamanczyk, R. and Vercaemer, C: “Method for Obtaining Gelled Hydrocarbon Compositions According to Said Method and Their Application the Hydraulic Fracturing of Underground Formations,” U.S. Pat. No. 4,507,213 (Mar. 26, 1985); McCabe, M. A., Terracina, J. M. and Kunzl, R. A.: “Continuously Gelled Diesel Systems for Fracturing Applications,” paper CIM/SPE 90-93, presented at the Petroleum Society of CIM/SPE International Technical Meeting, Calgary, Alberta, Canada (Jun. 10-13, 1990).; Huddleston, D A.: “Liquid Aluminum Phosphate Salt Gelling Agent,” U.S. Pat. No. 5,110,485 (May 5, 1992), a rapidly thickening gel composition can be achieved. With this chemistry, the aluminum source and phosphate ester can be added to the hydrocarbon as it is pumped downhole. The gel is formed on the way to the perforations. The expense of premixing the gel is eliminated, as well as the disposal problem if there is any unused gel.
Gelled oil systems are currently used primarily for fracturing and sand control applications. They are also used for coiled tubing (CT) cleanout applications, especially in water sensitive formations. Because of the low sand suspension capabilities of conventional gelled oils when pumping at high rates, foaming of the fluids is often recommended. However, gelled oils are difficult to foam, and often require fluorosurfactant compounds that are not environmental friendly. In addition, friction loss experienced with conventional gelled oil fluids is generally higher than that experienced with water based fluids, especially in turbulence. Some conventional gelled oils may tend to stick to tubing walls. In addition, the properties of conventional gelled oils are sensitive to the choice of the base oil and to the amounts of the aluminum complex and the phosphate ester.
There is a need for gelled oils that have increased viscosity and solids suspension capacity, less sensitivity to the concentration of components and to the nature of the base oil, and that are easy to foam.